The present invention relates to a high frequency dielectric filter device having a plurality of coaxial dielectric resonators arranged in parallel with each other.
There have been proposed various high frequency dielectric filter devices in which a dielectric substrate is provided with a plurality of through-holes which are formed in parallel with each other, each of the through-holes has a conductor layer arranged on its inner surface, and the dielectric substrate has an outer surface provided with an outer conductor which is connected with the inner conductor layers of the through-holes on one end surface of the dielectric substrate thereby to form a short-circuit surface, while the other end surface of the dielectric substrate is used as an open-circuit end surface. The high frequency dielectric filter devices having such an arrangement as stated above are used as a filter for a high frequency band. In this structure, a coaxial dielectric resonator is provided for each through-hole and thus a plurality of such resonators are arranged in parallel with each other.
An example of such conventional high frequency dielectric filter is shown in FIG. 1 of the accompanying drawings in which a dielectric substrate A is provided with two coaxial dielectric resonators B which are juxtaposed to each other. Each dielectric resonator B comprises a through-hole extended from an open-circuit end surface C to a short-circuit surface (not shown) of the dielectric substrate A and provided with a conductor layer D. In order to couple coaxial dielectric resonators B with each other, a conventional means can be used as by, for example, forming a hole E or a slit penetrating from the open-circuit end surface C to the short-circuit surface of the dielectric substrate A as shown in FIG. 1 or providing a counter-bore on the end portion of the formed coupling hole to enlarge the diameter of the portion.
With this kind of coupling, after the dielectric substrate is firstly formed to have a predetermined shape and inner and outer conductor layers are formed therein. Such a hole E or counter-bore should be formed at a high accuracy, with their widths and depths being decided, so that the production yield is low and difficulties are encountered in the manufacture of filters having equal characteristics. Further, the degree of coupling cannot be easily adjusted if it widely vary. In addition, there is another problem that the mechanical strength is deteriorated if the coupling hole is formed in the manner as stated above.
Meanwhile, in a case where a high frequency dielectric filter is adapted to telecommunications equipment such as portable wireless telephone or automobile telephone, it is often required that an attenuation pole should be provided at a frequency zone or position which is outside the intended frequency band and is apart from the center frequency by a predetermined frequency for isolating a transmitting signal from a receiving signal. To this ends, various arrangements have been attempted to satisfy this requirement. However, conventional means rely mainly on an external means such as a reactance element or the like for coupling coaxial dielectric resonators with each other and causes a problem in that the number of components is increased.